Modification of si and mg2si second phase in al alloys

ABSTRACT

A METHOD OF CHANGING THE MORPHOLOGY OF SI AND MG-SI SECOND PHASE IN AL ALLOYS BY ADDING LI AND AS TO THE AL ALLOY MELT.

United States Patent 3,627,518 MODIFICATION OF Si and Mg Si SECOND PHASE IN Al ALLOYS Garth D. Lawrence and George S. Foerster, Midland,

Mich., assignors to the Dow Chemical Company, Midland, Mich. No Drawing. Filed Sept. 26, 1969, Ser. No. 861,459 Int. Cl. C22c 21/00, 21/04 US. Cl. 75l47 4 Claims ABSTRACT OF THE DISCLOSURE A method of changing the morphology of Si and Mg Si second phase in Al alloys by adding Li and As to the Al alloy melt.

BACKGROUND OF THE INVENTION Al alloys containing Si and/or Mg Si (magnesium silicide) which are sand or permanent mold cast, have a microstructure of an Al matrix with needle or plate-like, interconnected second phase particles of Si or Mg Si. This dendritic type second phase structure is detrimental to castability and alloy performance. Thus it is desirable to be able to change or refine the physical structure (morphology) of the Si and Mg- Si particles.

Such change can in the case of Al-Si alloys be accomplished by the addition of another element to a melt of the alloy. For example, hypoeutectic Al-Si alloys (those alloys with higher Al than the Al-Si eutectic composition) are refined by the addition of small amounts of Na. P is commonly used with hypereutectic Al-Si alloys (those alloys with higher Si than the Al-Si eutectic composition). These additives produce a microstructure having nearspherical, non-connected Si particles.

To the best of our knowledge there is no known additive for changing the morphology of Mg Si second phase in Al alloys.

Therefore, an object of the present invention is to provide a method of changing the morphology of Si and Mg Si second phase particles in Al alloys.

THE INVENTION The method of the present invention comprises adding Li and As to an Al alloy melt containing Si and optionally Mg and solidifying the resultant melt, e.g. sand or permanent mold casting. The Li-As addition affects the nucleation and growth of the Si and/or Mg Si second phase particles on solidification to produce near-spherical, non-connecting second phase particles. Such change in morphology produces increased fluidity of the alloy during casting and improved mechanical properties of the alloy casting.

The amount of Li and As added is within the range of from about 0.01 to 2 weight percent each, based on the total weight of alloy melt plus additives. Preferably each should be added in an amount from about 0.1 wt. percent to 1.0 wt. percent.

The Al alloys upon which the Li-As addition is effective include any Al alloy containing substantial proportions of Si and/or Mg Si, for example, Al alloys containing from about 2 to 40 wt. percent Si and/ or Mg Si. Other elements, such as Zn, which do not affect the structure of the second phase can also be present in the alloys.

3,627,518 Patented Dec. 14, 1971 "ice In the practice of the present invention an Al alloy melt containing Si and optionally Mg is prepared. The Li and As are added in elemental form, or by way of master alloys or other appropriate means, and admixed with the melt. The melt is then cast or otherwise solidified in conventional manner.

The following examples are representative of the present invention. The percentages stated herein are weight percent based on the total weight of the composition unless otherwise indicated.

Examples 14 The following four Al alloys melts were prepared:

(1) Al-6.I% Mg.-3.5% Si (hypoeutectic AI-Mg Si alloy) (2) Al-12% Mg-7% Si(hypereutectic Al-Mg Si alloy) (3) A1-33% Si (hypereutectic Al-Si alloy) (4) Al-8% Si (hypoeutectic Al-Si alloy) A portion of each melt was cast in cylindrical graphite molds-2 /2 inches inside diameter, 2 /2 inches deep. The castings were cooled at a rate of about 2 F./min. to produce a slow cooled microstructure.

Additions of 1% As and 0.25% Li were admixed with each of the remaining alloy portions. These portions were then similarly cast as described above.

The microstructure of each of the castings was observed. The second phase particles of Mg Si and/ or Si in the castings without additives were needle or plate-like in structure and often interconnected. The second phase particles in the castings containing Li and As were chunky, nearspherical, non-connected in structure. The addition of Li and As had changed the morphology of second phase particles in a hypoeutectic and hypereutectic Al-Si alloy and Al-MggSi alloy. That this transformation is beneficial is shown in the next example.

Example 5 An Al-33% Si melt was prepared and cast in a manner similar to those of Examples 14. To a portion of the melt was added 0.25 Li and 1.0% As prior to casting. The tensile strength of each casting was measured.

The alloy had a tensile strength (TS) of 3 ksi without additives but with the Li and As the TS increased to 15 ksi. This increase is attributed to the change in morphology of the Si second phase particles caused by the addition of Li plus As.

Examples 6-17 A series of Al-7% Si alloy melts were prepared. Varying amounts of Li and As were added and the resultant melts cast after holding the melts at temperature (1350 F.) for varying periods of time. The amounts of Li and As retained were analyzed and the microstructures observed to determine the relative amount of second phase modification. These microstructures were compared against the unmodified microstructure of a sample of AI-7% Si containing no additives and microstructure of a sample modified with Na, a conventional additive. The latter microstructure was considered good in that almost all the Si second phase was in the near-spheroidal form. Those samples with As and Li additives with comparable microstructures were rated accordingly. Samples with lesser but still significant degrees of modification were rated some or little modification in comparison to the Na additive sample. The results are tabulated in Table I.

TABLE I Holding Percent Percent tim e, Example As 1 L1 2 As/Ll hrs. Modification 0. 033 0. 02 1. 7 0. 1 Little. 0. 036 0.05 0. 73 0. 5 Good. 0. 033 0. 09 0. 3B 1. 5 D0. 0. 037 0. 08 0. 47 2. 5 Do. 0.012 0. 02 0. 62 3 3. 1 D 0. 018 0. l2 0. 15 0. 6 Do. 0. 031 0. 08 0. 38 1. 0 Some. 0. 104 0. 02 5. 2 2. 0 Little. 0. 118 0. 02 5. 9 2. Do. 0. 082 0. 22 0. 38 3. 0 Good. 0. 030 0. 50 0. 06 3 3. 3 0.011 O. 60 0. 02 3. 6

1 Determined by neutron activation analysis.

2 Determined by speetrographic analysis.

3 C12 fluxing of melt {or 20 minutes.

4 Good but had formation of Li-Si second phase.

Although in each case of Li-As addition modification occurred, the additives were most effective when added within a As/Li ratio range of 1:1 to 1:10. Ratios greater than 1:1 produce lesser amounts of modification (Examples 6, 13, 14). Ratios less than 1:10 produce good modification but also form a Li-Si second phase, which could be detrimental in other regards (Examples 16 and 17). It is also significant that the beneficial effect of Li-As addition was maintained even after holding the melt at temperature for 2 to 3 hours and/or C1 fiuxing of the melt.

What is claimed is:

1. A method which comprises:

(a) adding Li and As to an Al alloy melt containing Si and optionally Mg, the Li and As each in an amount within the range of from about 0. 01 to 2 weight percent based on total weight of melt plus additives; and

(b) solidifying said melt to produce near-spherical, non-connecting Si or Mg Si second phase particles.

2. The method of claim 1 wherein Li and As are added each in an amount within the range of from about 0.1 to 1 weight percent.

3. The method of claim 1 wherein Li and As are added Within the As to Li weight ratios of 1:1 and 1: 10.

4. An alloy having near-spherical, non-connecting Si or Mg Si second phase particles consisting essentially of from about 0.01 to 2 wt. percent Li, from about 0.01 to 2 wt. percent As, Si and optionally Mg Si in a total amount of from about 2 to Wt. percent, and balance aluminum.

References Cited FOREIGN PATENTS 616,413 1/1949 Great Britain -147 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 

